Conventional processes for the recovery of high purity aromatic hydrocarbons such as benzene, toluene and xylenes (BTX) from various hydrocarbon feeds including catalytic reformate, hydrogenated pyrolysis gasoline, etc., utilize an aromatic selective solvent. Typically, in the practice of such processes, a hydrocarbon feed mixture is contacted in an extraction zone with an aqueous solvent composition which selectively dissolves the aromatic components from the hydrocarbon feed, thereby forming a raffinate phase comprising one or more non-aromatic hydrocarbons, and an extract phase comprising solvent having aromatic components dissolved therein.
It is generally desirable in aromatics extraction processes to use solvents that have high selectivity for the solute components as well as good solvency, or capacity. Generally, the higher the selectivity of a solvent, the higher the aromatic purity of the product produced. Often, however, it is found that solvents with high selectivity tend to have low solvency and solvents with high solvency tend to have poor selectivity. Accordingly, the choice of a particular solvent usually involves a compromise insofar as the above-identified properties are concerned.
There are a variety of solvents that have been proposed for aromatics extraction. These aromatic selective solvents generally contain one or more organic compounds containing in their molecule at least one polar group, such as a hydroxyl, amino, cyano, carboxyl or nitro radical. In order to be effective, the organic compounds of the solvent composition having the polar radical should have a boiling point substantially greater than the boiling point of the aromatic hydrocarbons to be extracted. In general, the solvent should also have a boiling point greater than the end boiling point of the aromatic component to be extracted from the hydrocarbon feed mixture. Typical solvents often comprise organic-containing compounds selected from aliphatic and cyclic alcohols, cyclic monomeric sulfones, the glycols and glycol ethers, as well as glycol amines, glycol esters and glycol ether esters. Some specific examples of aromatic extraction solvents commonly employed include; ethylene glycol, diethylene glycol, triethylene glycol, diglycolamine, tetraethylene glycol, dimethyl sulfoxide, sulfolane, acetonitrile, furfural, n-formyl morpholine, 3-methyl sulfolane, dimethyl formanide, phenol, methylethylketone, nitrobenzene and n-methyl pyrrolidone.
One type of aromatics extraction solvent of particular interest is the mixed extraction solvent described in U.S. Pat. Nos. 4,498,980 and 4,781,820. This mixed extraction solvent is comprised of a solvent component and a cosolvent component. The solvent component comprises the low molecular weight polyalkylene glycols of the formula: EQU HO--[CHR.sub.1 --(CR.sub.2 R.sub.3).sub.n --O].sub.m --H
wherein n is an integer from 1 to 5 and is preferably the integer 1 or 2; m is an integer having a value of 1 or greater, preferably between about 2 to about 20 and most preferably between about 3 and about 8; and wherein R.sub.1, R.sub.2 and R.sub.3 may be hydrogen, alkyl, aryl, aralkyl or alkylaryl and are preferably hydrogen and alkyl having between 1 and about 10 carbon atoms and most preferably are hydrogen.
The "cosolvent" component is a glycol ether of the formula: EQU R.sub.4 O--[CHR.sub.5 --(CHR.sub.6 --)--.sub.x O].sub.y --R.sub.7
wherein R.sub.4, R.sub.5, R.sub.6 and R.sub.7 may be hydrogen alkyl, aryl, aralkyl, alkylaryl and mixtures thereof with the proviso that R.sub.4 or R.sub.7 are not both hydrogen.
The above-identified patents disclose that the mixed extraction solvent is used in an extraction zone wherein the temperature is generally at least about 150.degree. C. (302.degree. F.) and is generally in the range of from about 150.degree. C. (302.degree. F.) to about 275.degree. C. (527.degree. F.). The patents further disclose that the mixed extraction solvent provides a certain unique balance of desirable characteristics including: (a) high selectivity for the aromatic feed components at the extraction temperatures; (b) high solvent capacity for the aromatic feed components at the extraction temperatures; (c) low capacity for the aromatic feed components at temperatures below the extraction temperatures; (d) chemical and thermal stability under the process conditions; (e) adaptability to a wider range of feeds; and (f) the solvent and cosolvent are sufficiently miscible to permit their recycle as a single recycled component.
The processes to which the use of the mixed extraction solvent has been directed are typically those wherein a bulk separation is desired, such as in the dearomatization of lube oil fractions. U.S. Pat. No. 4,781,820 specifically discloses a process for the dearomatization of a mixed hydrocarbon feed with low energy consumption in a continuous solvent extraction-solvent separation process that does not require the use of energy intensive downstream separation equipment, i.e., distillation, to recover the solvent from the aromatic product.
However, in some instances when high purity aromatics are desired, e.g., nitration grade aromatics, the energy intensive downstream separation equipment may be required in order to provide the desired product purity. Processes that utilize distillation operations for downstream separation typically employ an extractive distillation step to remove non-aromatic hydrocarbons from the rich solvent from the extractor followed by a steam distillation step to remove the aromatic hydrocarbons from the solvent.
One such process for producing hihg purity aromatics is described in U.S. Pat. No. 3,714,033 and provides for the use of a single distillation column wherein both extractive distillation and a steam stripping occur. The patent discloses the preferred use of a polyalkylene glycol solvent in a temperature range of from about 100.degree. C. (212.degree. F.) to about 200.degree. C. (392.degree. F.) to provide a high purity aromatics product.
Another process for producing high purity aromatics is described in U.S. Pat. No. 4,058,454 and provides for the use of extractive and steam distillation in separate columns. A particularly suitable class of solvents for use in the above-identified patent are those commonly referred to as the sulfolane type. The process utilizes an extraction temperature, with a sulfolane solvent, in the range of from about 80.degree. to about 400.degree. F. and can provide a high purity aromatic product.
In a number of instances where the production of high purity aromatics is desired, extraction operations are run at higher temperatures than the subsequent separation to remove the extract from the solvent. Three such references are U.S. Pat. No. 3,714,033 to Somekh, wherein the extract is cooled as it leaves the extractor to a temperature in the range of about 100.degree.-125.degree. C. before it enters a distillation zone. In a second reference, U.S. Pat. No. 3,431,199 to Reni, operates a multi-stage extractor high temperature with a high boiling organic selective solvent. After cooling the extract from the extraction column, the extract is conveyed to a decanter wherein two liquid phases are separated. The third reference, U.S. Pat. No. 4,498,980 to Forte employs a liquid/liquid extraction system based on a mixed extraction solvent wherein the aromatic rich solvent phase is cooled to promote the formation of two phases before it is brought to a distillation zone. Thus, each of the above references mandate that a rich solvent from the extraction zone must be cooled in order to separate the solvent from the aromatic and non-aromatic hydrocarbons. This is in complete contrast to the current invention where it was found to be desirable to heat the rich solvent stream before subsequent distillation and separation steps.
Not infrequently it is desired by refiners and other users of the above-described types of high purity aromatics extraction processes to increase the capacity or throughput of the units. Moreover, it is also desired to at least maintain or preferably improve the aromatics product purity. Hence, there is a need for aromatics extraction processes that utilize solvents having high selectivity and capacity which can provide a high purity aromatics product at high throughputs.